(1) Field of the Invention
The present invention relates to a process in which a spherical electrophotographic toner containing a colorant and having a particle size in a range suitable for a toner is directly prepared in the polymerization process for formation of a binder resin.
Furthermore, the present invention relates to an electrophotographic toner which is excellent not only in the flowability but also in various photographic characteristics.
(2) Description of the Prior Art
In the electrophotographic process, a toner is used for visualizing an electrostatic image. Toner particles are composed of a composition comprising a binder resin and, incorporated therein, a colorant and other additives such as a charge controlling agent, and have a particle size adjusted to a certain level, for example, 1 to 30 .mu.m. A resin having desirable electroscopic property and binding property, for example, a styrene resin, is used as the binder resin, and carbon black or other organic or inorganic coloring pigment is used as the colorant.
A most typical process for the production of an electrophotographic toner comprises melt-kneading a binder resin as described above with a colorant, cooling and pulverizing the molten composition and classifying the pulverized composition to recover a fraction having a certain particle size. The yield of the toner obtained through pulverization and classification is low and a large equipment is necessary for these operations. Accordingly, the manufacturing cost of the toner is very high. Moreover, the shape of the obtained particles is irregular and the flowability of the toner is generally low, and the toner is defective in that blocking is readily caused.
Various trials have been proposed to prepare toners directly in the polymerization process for formation of a binder resin. According to a typical instance, in a water-insoluble monomer is dissolved a polymerization initiator soluble in the monomer, additives such as a colorant are added to the solution, the resulting composition is suspended under high-speed shear stirring into an aqueous solution comprising an appropriate dispersant, for example, a water-soluble polymer, an inorganic powder and a surface active agent, and the suspension is subjected to polymerization to form colored polymer particles. According to this suspension polymerization process, however, the particle size of the final toner is determined by the state of suspension of the monomer composition in water, and the particle size distribution of the toner particles is very broad and readily influenced by the state of throwing of the monomer composition into water or by the stirring condition, and it is very difficult to obtain a toner uniform in the particle size. Furthermore, according to this suspension polymerization process, in general, only coarse particles having a size of scores of .mu.m to several mm are formed, and particles having a size of 1 to 30 .mu. m, suitable for a toner of a developer, can hardly be obtained. Of course, it is possible to reduce the particle size of the formed toner if the amount incorporated of the dispersant is increased. However, in this case, the dispersant is contained in the toner and the toner becomes sensitive to the moisture, and the electrophotographic characteristics are readily degraded. A particular post treatment is necessary to eliminate this disadvantage, and the step number is increased and the process cannot be practically worked.
A so-called emulsion polymerization is known as the process for preparing polymer particles having a relatively uniform particle size. This emulsion polymerization process, however, is defective in that the obtained particles are too fine and have a particle size smaller than 1 .mu.m and the step of removing the emulsifier after the polymerization is necessary. Therefore, according to this process, it is difficult to obtain a colored resin for a toner directly in the polymerization process.
Furthermore, there is known a process in which a styrene type or acrylic monomer having a polar group such as an amino group or a hydroxyl group and a colorant are subjected to suspension polymerization in the presence of an acid-soluble inorganic compound and the formed suspension is treated with an acid to obtain a toner composed of colored polymer fine particles (see Japanese Patent Publication No. 51-14895). This process, however, is defective in that a troublesome operation of dissolving and removing the inorganic compound becomes necessary and a part of the monomer is polymerized in the state dissolved in the aqueous phase at the dispersing and polymerizing steps to form as by-products fine polymer particles having a size smaller than 1 .mu.m, as in the so-called emulsion polymerization.